An industrial fan has as its basic components the hub and a blade. Smaller fans, up to about 42 inches in diameter, can be manufactured from a single casting. It is not economically feasible, however, to manufacture larger fans from a single casting.
There have developed therefore over the years different systems for the mounting of blades to a hub. For example, in a fixed-blade fan, the blades have been welded to the hub surface. This construction requires machining of the parts, precision fixtures to hold the blades in the proper position relative to the hub during welding and the cost of the welding operation which must be performed by a skilled welder. The welded fan, as a result, is costly and labor intensive.
The larger fans are normally formed by bolting the blades to the hub, whether the fan be a fixed or adjustable-pitch-type fan. In such fans, the hub is formed with a precisely machined bore for each blade in the surface of the hub. Correspondingly, each blade has at its base a precisely machined cylindrical shank which is inserted into the bore. The fit of the shank in the bore must be held to about 0.002 inch. In a fixed-blade fan of the type described, a bolt passes through the hub and the shank to hold the blade securely on the hub. More often, the innermost portion of the shank, extending into the inside of the hub, is threaded on the outside and serves as a bolt unto which a nut is secured. In an adjustable pitch blade, the hub may be split transversely to the axis of the hub and bolted together. The faces of the two hub halves have to be machined so that they will fit against each other when closed. This is a considerable area to be machined, so this is another expensive machining operation. A set screw engages an annular groove in the shank to secure the shank and hence the blade to the hub. To vary the pitch, the hub bolts are loosened to separate the hub halves slightly. When the set screw is also loosened, the pitch of each blade can be changed. Thereafter, all bolts are tightened to secure the blade to the hub.
In fans of the type described, the bearing surface of the blade with respect to the hub is small. This results in a significant stress concentration at this annular area, thus creating the area where fatigue and failure are likely to occur.
Other forms of mounting multiple pitch blades to hubs are disclosed in U.S. Pat. Nos. 3,357,496 and 3,545,884. All of these forms of fans, including the form specifically described above, require precise machining of a bore in the hub and of the shank of the fan blade which is to be fitted in the bore.
Another form of adjustable-pitch fan is disclosed in U.S. Pat. No. 3,300,123. That patent discloses a thin-walled sheet metal blade mounted on a spherical surface of a hub. The fan of that patent does not appear to be the heavy duty industrial fan to which the present invention is directed.